This invention relates to semiconductor light emitting devices and fabricating methods therefore, and more particularly to packaging and packaging methods for semiconductor light emitting devices.
It is known to provide semiconductor light emitting device type light sources in packages that may provide protection, color selection, focusing and the like for light emitted by the light emitting device. For example, the light emitting device may be a light emitting diode (“LED”). Various problems may be encountered during packaging of a power LED for use as a light source. Examples of such possible problems will be described with reference to the cross-sectional illustrations of a power LED in FIGS. 1 and 2. As shown in FIGS. 1 and 2, a power LED package 100 generally includes a substrate member 102 on which a light emitting device 103 is mounted. The light emitting device 103 may, for example, include an LED chip/submount assembly 103b mounted to the substrate member 102 and an LED 103a positioned on the LED chip/submount assembly 103b. The substrate member 102 may include traces or metal leads for connecting the package 100 to external circuitry. The substrate 102 may also act as a heatsink to conduct heat away from the LED 103 during operation.
A reflector, such as the reflector cup 104, may be mounted on the substrate 102 and surround the light emitting device 103. The reflector cup 104 illustrated in FIG. 1 includes an angled or sloped lower sidewall 106 for reflecting light generated by the LED 103 upwardly and away from the LED package 100. The illustrated reflector cup 104 also includes upwardly-extending walls 105 that may act as a channel for holding a lens 120 in the LED package 100 and a horizontal shoulder portion 108.
As illustrated in FIG. 1, after the light emitting device 103 is mounted on the substrate 102, an encapsulant material 112, such as liquid silicone gel, is dispensed into an interior reflective cavity 115 of the reflector cup 104. The interior reflective cavity 115 illustrated in FIG. 1 has a bottom surface defined by the substrate 102 to provide a closed cavity capable of retaining a liquid encapsulant material 112 therein. As further shown in FIG. 1, when the encapsulant material 112 is dispensed into the cavity 115, it may wick up the interior side of the sidewall 105 of the reflector cup 104, forming the illustrated concave meniscus.
As shown in FIG. 2, a lens 120 may then be placed into the reflective cavity 115 in contact with the encapsulant material 112. When the lens 120 is placed in the cavity 115, the liquid encapsulant material 112 may be displaced and move through the gap 117 between the lens 120 and the sidewall 105. The encapsulant may, thus, be moved out onto the upper surface of the lens 120 and/or upper surfaces of the sidewall 105 of the reflector cup 104. This movement, which may be referred to as squeeze-out, is generally undesirable for a number of reasons. In the depicted package arrangement, the lens will sit on a lower shelf if the encapsulant is not cured in a domed meniscus shape prior to the lens attach step. This may cause the lens to not float during thermal cycling and fail via delamination of encapsulation to other surfaces or via cohesive failure within the delamination, both of which may affect the light output. The encapsulant material or gel is generally sticky and may interfere with automated processing tools used to manufacture the parts. Moreover, the gel may interfere with light output from the lens 120, for example, by changing the light distribution pattern and/or by blocking portions of the lens 120. The sticky gel may also attract dust, dirt and/or other contaminants that could block or reduce light output from the LED package 100. The gel may also change the shape of the effective lens, which may modify the emitted light pattern/beam shape.
After placement of the lens 120, the package 100 is typically heat-cured, which causes the encapsulant material 112 to solidify and adhere to the lens 120. The lens 120 may, thus, be held in place by the cured encapsulant material 112. However, encapsulant materials having a slight shrinkage factor with curing, such as a silicone gel, generally tend to contract during the heat curing process. In addition, the coefficient of thermal expansion (CTE) effect generally causes higher floating of the lens at elevated temperatures. During cool-down, parts have a tendency to delaminate. As the illustrated volume of encapsulant beneath the lens 120 shown in FIG. 2 is relatively large, this contraction may cause the encapsulant material 112 to delaminate (pull away) from portions of the package 100, including the light emitting device 103, a surface of the substrate 102, the sidewalls 105 of the reflector cup 104 and/or the lens 120 during the curing process. The delamination may significantly affect optical performance, particularly when the delamination is from the die, where it may cause total internal reflection. This contraction may create gaps or voids 113 between the encapsulant material 112 and the light emitting device 103, lens 120, and/or reflector cup 104. Tri-axial stresses in the encapsulant material 112 may also cause cohesive tears 113′ in the encapsulant material 112. These gaps 113 and/or tears 113′ may substantially reduce the amount of light emitted by the light emitting device package 100. The contraction may also pull out air pockets from crevices (i.e., reflector) or from under devices (i.e., die/submount), which may then interfere with optical cavity performance.
During operation of the lamp, large amounts of heat may be generated by the light emitting device 103. Much of the heat may be dissipated by the substrate 102 and the reflector cup 104, each of which may act as a heatsink for the package 100. However, the temperature of the package 100 may still increase significantly during operation. Encapsulant materials 112, such as silicone gels, typically have high coefficients of thermal expansion. As a result, when the package 100 heats up, the encapsulant material 112 may expand. As the lens 120 is mounted within a channel defined by the sidewalls 105 of the reflector cup 104, the lens 120 may travel up and down within the sidewalls 105 as the encapsulant material 112 expands and contracts. Expansion of the encapsulant material 112 may extrude the encapsulant into spaces or out of the cavity such that, when cooled, it may not move back into the cavity. This could cause delamination, voids, higher triaxial stresses and/or the like, which may result in less robust light emitting devices. Such lens movement is further described, for example, in United States Patent Application Pub. No. 2004/0041222. The sidewalls 105 may also help protect the lens 120 from mechanical shock and stress.